The president emphasized that a quick intervention in the Syrian civil war was problematic, given the international considerations that should precede a military strike.Defense Secretary Chuck Hagel declined to discuss any specific force movements while saying that Obama had asked the Pentagon to prepare military options for Syria.US defense officials told The Associated Press that the Navy had sent a fourth warship armed with ballistic missiles into the eastern Mediterranean Sea but without immediate orders for any missile launch into Syria.US Navy ships are capable of a variety of military action, including launching Tomahawk cruise missiles, as they did against Libya in 2011 as part of an international action that led to the overthrow of the Libyan government."The Defense Department has a responsibility to provide the president with options for contingencies, and that requires positioning our forces, positioning our assets, to be able to carry out different options - whatever options the president might choose," Hagel told reporters traveling with him to Asia.Hagel said the US is coordinating with the international community to determine "what exactly did happen" near Damascus earlier this week.According to reports, a chemical attack in a suburb of the capital killed at least 100 people. It would be the most heinous use of chemical weapons since Iraqi leader Saddam Hussein gassed thousands of Kurds in the town of Halabja in 1988.Obama remained cautious about getting involved in a war that has killed more than 100,000 people and now includes Hezbollah and al-Qaida.He made no mention of the "red line" of chemical weapons use that he marked out for Syrian President Bashar Assad a year ago and that U.S. intelligence says has been breached at least on a small scale several times since."If the US goes in and attacks another country without a UN mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it - do we have the coalition to make it work?" Obama said Friday. "Those are considerations that we have to take into account."Obama conceded in an interview on CNN's "New Day" program that the episode is a "big event of grave concern" that requires American attention. He said any large-scale chemical weapons usage would affect "core national interests" of the United States and its allies. But nothing he said signaled a shift toward US action.U.S. defense officials spoke on condition of anonymity because they weren't authorized to discuss ship movements publicly. But if the U.S. wants to send a message to Assad, the most likely military action would be a Tomahawk missile strike, launched from a ship in the Mediterranean.For a year now, Obama has threatened to punish Assad's regime if it resorted to its chemical weapons arsenal, among the world's vastest, saying use or even deployment of such weapons of mass destruction constituted a "red line" for him.A US intelligence assessment concluded in June chemical weapons have been used in Syria's civil war, but Washington has taken no military action against Assad's forces.U.S. officials have instead focused on trying to organize a peace conference between the government and opposition. Obama has authorized weapons deliveries to rebel groups, but none are believed to have been sent so far.In his first comments on Syria since the alleged chemical attack, Obama said the U.S. is still trying to find out what happened.Hagel said Friday that a determination on the chemical attack should be made swiftly because "there may be another attack coming," although he added that "we don't know" whether that will happen.After rebels similarly reported chemical attacks in February, U.S. confirmation took more than four months. In this instance, a U.N. chemical weapons team is already on the ground in Syria. Assad's government, then as now, has rejected the claims as baseless.Obama also cited the need for the U.S. to be part of a coalition in dealing with Syria. America's ability by itself to solve the Arab country's sectarian fighting is "overstated," he said.